Art of mounting vibratile bodies



Sept. 2, 1941- l. E. GROSDOFF 2,254,843

ART OF MOUNTING VIBRATILE BODIES Filed June 29, 1940 HL'FME'I'I CELL Y SEHLED EN VELOPE 6 /76 P 72! -56 LIFE DETERMINED By FOFMUL 19 Zmventor {901" H Grosdof Patented Sept. 2, 1941 UNITED ART OF MOUNTING VIBRATILE BODIES Igor E. Grosdoff, Upper Darby, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application June 29, 1940, Serial No. 343,134

3 Claims.

My invention relates'to the art of mounting vibratile bodies such, for example, as piezoelectric crystals, tuning forks, etc., and has for its principal object to provide means to compensate for the tendency of such bodies to change frequency when subjected to changes in temperature.

It may be said generally that the frequency of a vibrating body such as a quartz crystal or a tuning fork depends upon the temperature at which it is operated, that is to say, any change in the operating temperature is usually followed by a change in frequency. Several methods have heretofore been proposed for decreasing or compensating for such changes or drifts in frequency with temperature. One such method involves the use of a crystal of an orientation or cut having an inherently low temperature coefiicient of frequency. This method as applied to tuning forks involves the use of alloys, the form or ingredients of which are selected with a view of endowing the fork with a desired low temperature coeflicient of frequency. The zero or other desired low temperature coefficient of frequency is seldom exactly achieved by such methods so that resort is usually made to mounting the fork or crystal in a thermostatically controlled oven in order to make the frequency drift as small as possible.

My present invention dispenses with the use of cumbersome and expensive constant temperature apparatus and provides a hermetically sealed unheated tube or envelope in which the Vibratile element is mounted.

My invention is predicated upon the fact that the pressure of an ambient medium (such as air or gas) of constant volume may be utilized to alter the temperature-frequency characteristic of a vibratile element. More specifically, I have discovered that a vibratile element mounted in a hermetically sealed envelope containing air or other gas at a pressure determined by the natural temperature-frequency characteristic of said element and dictated by a certain formula (hereinafter set forth) will exhibit azero or some desired low positive or negative temperature coefficient of frequency.

In connection with the above, let us write a general equation for the effect of gas pressure on the frequency of an oscillating body:

fPZ is the frequency of said body at pressure P2, in is the frequency of said body at pressure P1,

and

K is a coefficient representing change in frequency per unit change in pressure. K is positive when P2 P1, i. e., when increase in pressure results in a decrease in frequency.

The effect of temperature on the frequency of the same vibrating body is given by:

where:

ft2 is the frequency of said body at temperature t2 )m is the frequency of said body at temperature t1, and

a is a coeificient representing change in frequency per unit change in temperature. a is positive when t2 t1, i. e., when an increase in temperature results in an increase in frequency.

The law of gases showing the relation between the temperature and pressure at constant volume is:

P12 is gas pressure at temperature 152 Pu is gas pressure at temperature t1, and B is a gas constant equal to approximately .00366.

The left side of this Equation 8 represents the net change in frequency due to changes in temperature and pressure. If the net change in frequency desired is zero, then:

since 1=10PX .00366 or, solved: P1=27.3 lbs. pressure.

Accordingly, a gas pressure in the envelope T (Fig. 1) in which the vibratile element E is mounted should, in this case, be substantially twice atmospheric pressure to ensure a substantially zero temperature coefiicient of frequency.

As indicated in the foregoing example, it is desirable to start with a vibratile element having a naturally low, preferably positive, temperature coefiicient of frequency in order to avoid the necessity of using unduly high gas pressures, but this preferred procedure in no wise detracts from the factor of merit of the invention since the principal difficulty heretofore confronting the art has been that of obviating relatively small changes in frequency with temperature.

I am cognizant of the fact that for various reasons others have heretofore proposed mounting a piezoelectric crystal in a hermetically sealed vacuum or gas tube, but, as far as I am aware, no one up to now has proposed to correlate the pressure within the envelope with the natural temperature coeflicient of frequency of the vibratile element to achieve another or overall temperature-frequency characteristics.

In the accompanying drawing, I have shown a vibratile element comprising a quartz piezoelectric element E mounted in a conventional airgap holder consisting of two spaced metal plates or electrodes L, L and enclosed in a hermetically sealed envelope T which will be understood to contain a filling of a suitable gas at a pressure dictated by the formula of my invention.

My invention is obviously not limited to any particular form of hermetic closure, nor to the use of a filling of any one gas.

What I claim is:

1. Method of mounting a vibratile element in an hermetically sealed envelope to cause said element to exhibit a desired temperature-frequency characteristic, said method comprising causing the pressure of the ambient within said envelope to vary with the tendency of the said element to change frequency with changes in temperature and in agreement with the formula or I@ where: P is the pressure of the ambient within said envelope, 0c is the natural temperature coeflicient of frequency of said element, K is the coefficient representing changes in frequency of said element, per unit change in pressure, and B is equal to substantially .00366.

2. In combination, a vibratile element and a hermetically sealed envelope in which said element is mounted, the pressure of the ambient in said envelope being so related to the natural temperature coefficient of frequency of said vibratile element that it exhibits an overall temperature coeflicient of a desired value.

3. In combination, a vibratile element, and a hermetically sealed envelope in which said element is mounted, the pressure of the ambient in said envelope being that dictated by the formula C! K B where: P is the pressure of the ambient in said envelope, or is the natural temperature coefficient of frequency of said element, K is a coefficient representing changes in frequency of said element per unit change in pressure, and B is equal to substantially .00366, whereby said element exhibits an overall temperature coefficient of frequency of a desired value.

IGOR E. GROSDOFF. 

